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#前面提到的操作函数和语句块是传统的面向过程编程，而编写大型程序，通常采用面向对象编程。类和对象是面向对象编程的两个主要方面，类创建一个新类型，而对象是类的实例。Python没有什么值类型与引用类型之分，它把所有事物统统看作是类。类使用class关键字来创建。
#1. Self
#类的方法与普通的函数只有一个特别的区别——它们必须有一个额外的第一个参数名称，但是在调用这个方法的时候你不为这个参数赋值，Python会提供这个值。这个特别的变量指对象本身，按照惯例它的名称是self。
#你一定很奇怪Python如何给self赋值以及为何你不需要给它赋值。举一个例子会使此变得清晰。假如你有一个类称为MyClass和这个类的一个实例MyObject。当你调用这个对象的方法MyObject.method(arg1, arg2)的时候，这会由Python自动转为MyClass.method(MyObject, arg1, arg2)——这就是self的原理了。
#-------------------------------------------------------------------------------
#2. 类
#一个空类：
class Person:
    pass #An empty block
#类的使用：

p = Person()
print(p)
#-------------------------------------------------------------------------------
#3. 方法
#类/对象可以拥有像函数一样的方法，这些方法与函数的区别只是一个额外的self变量。

class Person:
    def sayHi(self):
        print("Hello, how are you?")
p = Person()
p.sayHi()
#注意调用sayHi方法时没有任何参数，但仍然在函数定义时有self。

#-------------------------------------------------------------------------------
#4. __init__方法
#在Python的类中有很多方法的名字有特殊的重要意义。像__init__，类似于构造函数。__init__方法在类的一个对象被建立时，马上运行。这个方法可以用来对你的对象做一些你希望的初始化 。注意，这个名称的开始和结尾都是双下划线。
class Person:
    def __init__(self, name):
        self.name = name
    def sayHi(self):
        print("Hello, my name is", self.name)
p = Person("known")
p.sayHi()
#-------------------------------------------------------------------------------
#5. 域
#Python有两种类型的域——类的变量和对象的变量，它们根据是类还是对象拥有这个变量而区分。
#类的变量由一个类的所有对象（实例）共享使用。只有一个类变量的拷贝，所以当某个对象对类的变量做了改动的时候，这个改动会反映到所有其他的实例上。
#对象的变量由类的每个对象/实例拥有。因此每个对象有自己对这个域的一份拷贝，即它们不是共享的，在同一个类的不同实例中，虽然对象的变量有相同的名称，但是是互不相关的。

class Person:
    '''Represnets a person.'''
    population = 0

    def __init__(self, name):
        '''Initializes the person's data.'''
        self.name = name
        print("(Initializing %s)" % self.name)

        #When this person is created, he/she adds to the population
        Person.population += 1

    def __del__(self):
        '''I am dying.'''
        print("%s says bye." % self.name)

        Person.population -= 1

        if Person.population == 0:
            print("I am the last one.")
        else:
            print("There are still %d people left." % Person.population)

    def sayHi(self):
        '''Greeting by the person.
          Really, that's all it does.'''
        print("Hi, my name is %s." % self.name)

    def howMany(self):
        '''Prints the current population.'''
        if Person.population == 1:
            print("I am the only person here.")
        else:
            print("We have %d person here." % Person.population)


swaroop = Person('Swaroop')
swaroop.sayHi()
swaroop.howMany()

kalam = Person('Abdul Kalam')
kalam.sayHi()
kalam.howMany()

swaroop.sayHi()
swaroop.howMany()

del kalam
del swaroop

#运行结果：
#(Initializing Swaroop)
#Hi, my name is Swaroop.
#I am the only person here.
#(Initializing Abdul Kalam)
#Hi, my name is Abdul Kalam.
#We have 2 persons here.
#Hi, my name is Swaroop.
#We have 2 persons here.
#Abdul Kalam says bye.
#There are still 1 people left.
#Swaroop says bye.
#I am the last one.
#-------------------------------------------------------------------------------

#6. 继承
#在类名后面跟一对圆括号，基类名写在圆括号内。

class SchoolMember:
    '''Represents any school member.'''
    def __init__(self, name, age):
        self.name = name
        self.age = age
        print("(Initialized SchoolMember: %s)" % self.name)

    def tell(self):
        '''Tell my details.'''
        print("Name:'%s' Age:'%s'" % (self.name, self.age))

class Teacher(SchoolMember):
    '''Represents a teacher.'''
    def __init__(self, name, age, salary):
        SchoolMember.__init__(self, name, age)
        self.salary = salary
        print("(Initialized Teacher: %s)" % self.name)

    def tell(self):
        SchoolMember.tell(self)
        print("Salary: '%d'" % self.salary)

class Student(SchoolMember):
    '''Represents a student.'''
    def __init__(self, name, age, marks):
        SchoolMember.__init__(self, name, age)
        self.marks = marks
        print("(Initialized Student: %s)" % self.name)

    def tell(self):
        SchoolMember.tell(self)
        print("Marks: '%d'" % self.marks)


t = Teacher("Mrs. Shrividya", 40, 30000)
s = Student("Swaroop", 22, 75)

print() # prints a blank line

members = [t, s]
for member in members:
    member.tell() # works for both Teachers and Students

#输出结果：
#(Initialized SchoolMember: Mrs. Shrividya)
#(Initialized Teacher: Mrs. Shrividya)
#(Initialized SchoolMember: Swaroop)
#(Initialized Student: Swaroop)

#Name:'Mrs. Shrividya' Age:'40'
#Salary: '30000'
#Name:'Swaroop' Age:'22'
#Marks: '75'